The bambara groundnut (Vigna subterranea (L.) Verdc.) is an annual, creeping legume with glabrous, trifoliate leaves. It has a deep taproot surrounded by lateral profuse roots bearing N-fixing nodules. Flowers are presented as papilionaceous racemes. Once fertilized, the flower is drawn below the soil through a tunnel. Fruits are 2 or 3-seeded pods, 1.5 to 3 cm long, wrinkled and at first indehiscent. Seeds are round, smooth and very hard when dry (Ecoport, 2009). Average yields are 300-800 kg/ha, but can range from 100 kg/ha up to 4 t/ha.

Vigna subterranea grows well where groundnut does not (Brink et al., 2006). It is cultivated primarily for its seeds, which are used as human food. In Africa, it is the third most commonly eaten legume after groundnut and cowpea (Omoikhoje, 2008). The seeds are used in many types of foods, some of which are an important part of human diets. Mature, dry seeds are boiled and eaten as a pulse. Seeds can be milled to make flour. Ripe seeds are very hard and must be cooked for longer than those of other legumes. Cream-coloured seeds are often preferred to red and black seeds, because they are less bitter and take less time to cook. Large seeds are preferred to smaller ones, especially for use as a snack. Smaller seeds are ground into flour for use in various recipes.

The processing of Vigna subterranea results in by-products (shells of offals) that can be fed to animals. Seeds can also be fed to animals if surplus to human requirements. The leafy shoots are used as fodder (Brink et al., 2006).

Distribution

Vigna subterranea originates from West Africa (Nigeria, Cameroon, Central African Republic and Chad) and is cultivated in drier tropical Africa. In Southern Africa, Zimbabwe is the centre of production. It can also be found in tropical parts of America, Asia and Australia but the present degree of cultivation outside Africa is basically negligible.

Vigna subterranea is adapted to hot, dry, marginal soils, from sea level to an altitude of 2000 m. It continues to grow in harsh conditions better than most crops (sorghum, maize and peanuts). Optimal conditions for growth are 30-35°C day-temperature for germination, average day temperatures ranging from 20 to 28°C under full sun, average annual rainfall of 600-750 mm (optimum yields are obtained when rainfall is higher), good P and K soil content and light sandy loams with a pH of 5.0-6.5. Vigna subterranea can grow in more humid conditions (annual rainfall > 2000 mm), and in every type of soil provided it is well drained and not too calcareous. It is tolerant to drought, to pests and diseases, particularly in hot conditions. In many traditional cropping systems it is intercropped with other root and tuber crops (Brink et al., 2006).

Environmental impact

Like other legume plants, Vigna subterranea is a good soil fertilizer and a good rotation crop. It does not require any additional fertilizer. It is generally intercropped with cereals (maize, sorghum, and pearl millet), other pulses (cowpea, groundnut), root and tuber crops, or vegetables.

Nutritional aspects

Nutritional attributes

Because of their relatively high protein content, Vigna subterranea seeds are a potentially valuable protein source for all classes of livestock. However, it has been recommended to use the shells and other by-products to feed animals and to use the seeds as food so that people can benefit from their protein and energy during the dry season. The essential amino acid profile of the seeds is comparable to that of soybean (Omoikhoje, 2008; Fetuga et al., 1975).

Potential constraints

Like most legume seeds, Vigna subterranea seeds contain antinutritional factors such as trypsin inhibitors, phytates and tannins, including condensed tannins in the seed coat. Tannin content is correlated with coat colour, the cream-coloured seeds containing less tannin than brown or black seeds. Tannins can be deleterious to livestock performance though they have beneficial nutritional properties in some cases (by-pass protein in ruminants, anthelmintic effect).

Vigna subterranea seeds are reported to have higher antitryptic activity than soybean and the level of activity depends on the variety (landrace) (Tibe et al., 2007). Heat treatments such as boiling or roasting are usually effective in destroying trypsin inhibitors.

Phytates are found in high proportions in Vigna subterranea seeds and are known to reduce cation availability (Ca particularly) (Nwanna et al., 2005).

In Nigeria, the possibility of including toasted bambara groundnut seeds in growing rabbit feeds was tested by incorporating graded levels up to 35% in complete diets. A non significant positive influence of the graded level of bambara groundnut seed meal was observed on the rabbit daily weight gain, feed efficiency, dressing percentage and meat-to-bone ratio. The best results were obtained with the 25% level. Rabbits carcass cut-up parts as well as internal organs (liver, heart, kidneys and lungs) were not influenced by the dietary levels of bambara groundnut meal (Joseph et al., 2000).

The necessity of toasting the seeds before utilisation in rabbit feeding was not demonstrated, and may be questionable because the antitrypsin factor, the main limiting factor present in many legume seeds such as soybean seeds, was proved to be thermo-resistant in bambara groundnut seeds (Benjakul et al., 2000). In addition, good growth performances were obtained with rabbits fed on raw (non-toasted) bambara groundnut wastes (Ani, 2007).